Cell rearrangement is responsible for more morphogenetic (shape) changes during embryogenesis than any other process, and is a key component of tumor cell metastasis. Oriented rearrangement of cells with an epithelium (cell sheet) is essential for elongation of a number of different tubular organs, such as gut and kidney. The Drosophila hindgut, homologous in many features to the vertebrate colon, is an excellent genetic model in which to study epithelial cell rearrangement. We have defined a transcriptional regulatory hierarchy that establishes the anterior domain of the hindgut, and have shown that expression in this domain of the Drosophila ligand for the JAK/STAT pathway is required for oriented cell rearrangement. Most recently, we have demonstrated that one of the Rho-family small GTPases, Rac, is required for hindgut cell rearrangement. We propose to characterize, at high resolution, cell rearrangement, particularly the protrusions between rearranging cells, in both fixed and living, wild-type and Rac mutant embryos. Our testable hypothesis is that spatially localized modulation of Rac activity via specific GEFs, and interaction of activated Rac with specific targets, is required to promote changes in cell shape and motility, and thereby mediolateral cell intercalation, in the hindgut. In the proposed analysis, we will identify and characterize these Rac-interactors in the hindgut by lossof- function genetic analysis of candidate genes, and by screening of a novel expression library for rescue of the Rac mutant hindgut phenotype. This will provide a unique, in vivo understanding of the molecular basis of oriented cell rearrangement. In the course of this work we will generate multiple genetically engineered Drosophila strains that will allow novel approaches to the analysis of organogenesis. This work, by providing insight into both cell rearrangement and tubule elongation, has important implications for both cancer therapy and tissue engineering. [unreadable] [unreadable] [unreadable]